Zinc oxide-poly(acrylic acid)surgical cements

ABSTRACT

DENTAL CEMENTS ARE PREPARED BY MIXING A SURGICAL GRADE METAL OXIDE POWDER WITH A WATER-SOLUBLE POLYMER OF ACRYLIC ACID AND WATER TO GIVE A PLASTIC MASS THAT RAPIDLY HARDENS. THE CEMENTS THUS PRODUCED HAVE GREATER ADHESION AND CAUSE LESS IRRITATION THAN CONVENTIONAL ZINC PHOSPHATE CEMENTS.

3,751,391 ZINC OXIDE-POLY(ACRYLIC ACID) SURGICAL CEMENT S U.S. Cl.26029.6 M 10 Claims ABSTRACT OF THE DISCLOSURE Dental cements areprepared by mixing a surgical grade metal oxide powder with aWater-soluble polymer of acrylic acid and water to give a plastic massthat rapidly hardens. The cements thus produced have greater adhesionand cause less irritation than conventional zinc phosphate cements.

This application is a continuation of my earlier tiled application Ser.No. 836,991, filed June 26, 1969, now US. Pat. 3,655,605, issued Apr.11, 1972, which is a continuation-in-part of my earlier filedapplication Ser. No. 692,711, [filed Dec. 22, 1967, now abandoned, bothentitled, Improvements Relating to Surgical Cements.

This invention relates to surgical cements and is particularly concernedwith the provision of an improved dental cement.

The materials known as dental cements have many applications indentistry including cementing restorations into place in the tooth,providing a base and/or lining in a tooth cavity, providing a temporaryfixing for the bands of orthodontic appliances to the teeth and sealingrootcanals after endodontic treatment. At the moment, zinc phosphatecements are widely used for this purpose. These cements are obtained bymixing together a zinc oxide powder and a buffered ortho-phosphoric acidsolution immediately prior to use. The ease of mixing and setting timeof the cement are influenced by the relative proportions andreactivities of the components. Such zinc phosphate cements are notentirely satisfactory because the cement is highly acidic in nature (pH1-2) and can cause pulpal injury if it is placed directly on freshly cutdentine. Furthermore, the cementing action of such materials is purelymechanical and there is little actual adhesion between the set zincphosphate cement and the tooth substance.

The present invention broadly provides an improved cement for dental andother surgical purposes prepared by mixing a surgical grade of metaloxide powder with an aqueous solution of a polyacrylic acid. This newcement retains the strength of the zinc phosphate cements but has theadvantage that it is much less irritant in nature and hence less painfulto the patient and furthermore, exhibits a greater degree of adhesion tothe tooth or other calcareous substances. This improved adhesion isbelieved to be the result of chelation between the poly acid in thecement and the calcium.

The present invention provides a surgical cement pack comprising asurgically acceptable grade of metal oxide powder, and an aqueoussolution containing at least 20% by weight and preferably 40 to 60% byweight of a polyacrylic acid having a viscosity determined averagemolecular weight of 5,000 to 250,000, the two components upon beingmixed together in a ratio between 0.5 :1 to 4:1 on a weight basisforming a plastic mass which United States Patent 3,751,391 PatentedAug. 7, 1973 ice rapidly hardens as a surgical cement but which remainsplastic long enough to be formed into a desired shape. The cementusually remains workable for up to about 8 minutes after which time ithardens rapidly in the shape into which it has been set.

The metal oxide powder may be similar to that used in the production ofthe conventional cements but is preferably further de-activated byheating. Setting time appears to be proportional to particle size andreactivity and can be prolonged by heating the powder particles; forexample a typical de-activating heat treatment of an -Analar zinc oxideinvolves heating powder at about 1000 C. for up to about 40 hours. Theconventional zinc oxide powder to be de-activated need not be zinc oxidebut may be mixed with a proportion of other metal salts or oxides toimprove the surgical properties of the final cement product. Theseadditives are already incorporated in the powders used for theproduction of conventional zinc oxide/phosphoric acid based cements. Thepredominant component is always zinc oxide but magnesium oxide, bismuthoxide, calcium phosphate and/ or calcium fluoride may also be added. Theterm surgical grade zinc oxide powder includes zinc oxide containingsuch conventional additives. Other metal oxides which may be usedinclude the oxides of cadmium, mercury, calcium, copper, strontium,barium and silver.

The polyacrylic acid should have a viscosity determined averagemolecular weight in the range of 5,000 to 250,000, and preferably from15,000 to 150,000, i.e. the polymer should be readily water-soluble.Ease of handling is related to the viscosity of the aqueous solution andif it is too viscous cobwebbing" becomes a problem when the desiredquantity of solution is removed from the container and mixed with themetal oxide powder. Solutions of polymers with molecular weights well inexcess of 150,000 can be obtained with acceptable viscosities but it isfound that for some purposes the concentration of the acid in suchsolutions is too low so that the final cement product does not havesufficient mechanical strength.

Accordingly, the solution of polyacrcylic acid used in this dentalapplication should preferably be of a high concentration and the presentinvention includes a solution containing at least 40% w./w. andpreferably 50 to 60% w./w. of a polyacrylic acid having a viscositydetermined average molecular weight of 15,000 to 150,000 and which canbe used in the manner described to obtain a surgical cement. Resultsobtained so far suggest that the optimum molecular weight of a polymershould be at least about 25,000 and preferably of the order of 50,000for maximum mechanical strength. Very good results are obtained with asolution of about 42% concentration and a molecular weight of about25,000 or about 55,000 to 80,000 according to clinical application.

Polymers having molecular weights of from 5,000 to 15,000 and from150,000 to 250,000 can be used in concentrations of from 20 to 60% byweight in applications where maximum strength is not required or notdesirable, for example periodontal packs or temporary fillings andcementation. *Polymers having molecular weights of from 15,000 to150,000 and concentrations in the range 20 to 40% by weight may also beuseful for these applications.

The polyacrylate cements are designed to be made by the practitionerimmediately prior to use as are the zinc phosphate dental cements andthe materials in the pack may be brought together, mixed and formed intothe required shape immediately prior to use. The mixed components form aplastic mass which can be cast, moulded, blown or otherwise formed intothe required shape during the brief period that the mixture retains itsplastic properties. It is preferred to use these cements in contact withcalcareous materials as particularly strong adhesion occurs to suchmaterials, apparently through the formation of chemical bonds.

A quantity of polyacrylic solution sufiicient to make up one mix ofcement may be easily withdrawn from its container using a glass rod orsimilar instrument or extruded from a tube or like container and thismay be mixed with a quantity of the oxide powder on a suitablesurface.The components mix quite rapidly to give a homogeneous mass whichcommences to harden within a few minutes and is usually completely hardwithin about eight minutes of mixing. In addition to the otherparameters mentioned above, the rate of hardening, strength of finalproduct, etc., are determined by the powder/ liquid ratio and ideallythis should be about 0.5:1 to 4:1 on a weight basis. Too little or toomuch powder normally results in a mixture that is diflicult to form intothe desired shape but it is not essential to prepare the cement using aratio within these limits. Careful matching of the powder and liquidcomponents will enable an acceptable plastic mass to be obtained whichwill harden in an acceptable time. Typical powder/liquid ratios thathave given rise to valuable cement products are 1:1, 1.5:1, 2:1 and3.25:1 and the optimum ratio for a particular powder and liquid may bedetermined by a few simple preliminary experiments which will normallyindicate a figure in the range 1.5 :l to 3.5: 1. Alternatively, the packmay contain a mixture of the components in the required ratio, providedthat one or both of the components are encapsulated to prevent prematurereaction.

The polyacrylic acid solution may be prepared by any of the customarilyused polymerisation techniques in aqueous solution for examplepolymerisation may be carried out in the presence of ammoniumpersulphate, and various chain transfer reagents to give solutionscontaining up to about 30% of the polymer. This solution may then beconcentrated, if necessary, to give more viscous solutions containingmore than 40% of the polymer which are used in the preparation of thepreferred dental cements. Use of weaker solutions of polymers oftenleads to the production of a product with lower mechanical strength.

A modified polymerisation technique has been developed which is believedto give rise to a polymer having a narrower molecular weight range thanthose obtained by the more conventional techniques. These narrowmolecular weight range polymers give rise to particularly good cementsand may be prepared by adding an aqueous solution of acrylic acid and anaqueous solution of ammonium persulphate, preferably slowly andcontinuously, to an aqueous solution of ammonium persulphate. Variousother monomers may be included in the polymerising system to giveacrylic acid copolymers having modified properties, provided that theacrylic acid copolymer is soluble in water and reacts with a metal oxidein the required manner.

The following examples are given to illustrate the invention.Temperatures are in C., quantities in parts by weight.

EXAMPLE 1 (A) Preparation of zinc oxide (a) Analar grade zinc oxide isheated in an electric furnace at 900-1000" C. for 12-24 hours until thedesired degree of reactivity in setting with one of the polyacrylicacids below has been achieved. The reactivity is measured by a settingtime determination at 37 C. in 100% RH. (relative humidity) in a similarmanner to the test described in B5. 3364 (Dental Zinc Phosphate Cement).

(b) Zinc carbonate is heated for 30 minutes at a temperature between350-600 C. The resulting zinc oxide is deactivated by heating at900-1000 C. as in (a) above.

A mixture of 90 parts Analar grade zinc oxide to parts by weight ofAnalar grade magnesium oxide is heated at 1000-1300 C. for 8-12 hours.The resulting cake A four-necked round bottomed flask is fitted with areflux condenser, gas inlet, dropping funnel and a stirrer. Thefollowing charge is placed in the flask and de-gassed with nitrogen:

Distilled water Ammonium persulphate 0.9

The solution is heated to 80-85 C. and 30 parts of redistilledinhibitor-free acrylic acid added over 4-5 hours. After the addition iscomplete the mixture is heated for a further hour at -110 C. Thesolution is then subjected to vacuum distillation to remove excess wateruntil the polyacrylic acid concentration is about 41 percent asdetermined by potentiometric titration. The molecular weight of thepolyacrylic acid determined by viscosity measurement is approximately80,000.

(C) Preparation of the polyacrylate cement The zinc oxide prepared as inExample 1A (a) is spatulated on a glass slab in the usual dental mannerwith the polyacrylic acid solution prepared as above in the ratio of3.25:1. A plastic mix results which sets to a hard strong mass (Table 1)which is suitable as a cavity liner, a cement base for other restorativematerials, or as a temporary filling material.

EXAMPLE 2 A polyacrylic acid solution, prepared as described in Example1B but of 50-55 percent concentration, is used to form a cement with apowder:liquid ratio of 2:1 as otherwise described in Example 1C. Thisstronger cement (Table 1) can be used for the purposes mentioned inExample 1 but also for the cementation of orthodontic bands andattachments to the teeth.

EXAMPLE 3 The polyacrylic acid solution of Example 2 is used to form acement with the zinc oxide described in Example 1A (c) in thepowder:liquid ratio of 15:1. The mix is suitable for the cementation ofgold inlays, crowns and bridges and porcelain jacket croums.

EXAMPLE 4 A polyacrylic acid solution is prepared according to themethod of Example 1B but with the following charge in the flask:

Distilled water 100 Isopropanol .l.. 100 Ammonium persulphate l0Re-distilled inhibitor-free acrylic acid, 100 parts, is added over 5-6hours the solution being maintained at 80-85 C. The solution is heatedfor a further hour at 100-110 C. and then concentrated by vacuumdistillation to give a polyacrylic acid, concentration 42% and viscositydetermined molecular weight approximately 26,000. This liquid is mixedas before with the zinc oxide of Example 1A (c) in a powder liquid ratioof 2.5 :1. This cement which gives a more fluid mix is suitable for thepurposes mentioned in Example 3.

EXAMPLE 5 A polyacrylic acid solution prepared according to theconditions of Example 4 but of 60% concentration is mixed with the zincoxide of Example 1A (a) in a powder liquid ratio of 1:1. This cement can'be employed for the purposes mentioned in Example 1.

EXAMPLE 6 A zinc oxide prepared according to Example 1A (c) is blendedin a rotary mixer with an equal part of a silicate glass containingleachable fluoride. The resulting powder is mixed with a 41% polyacrylicacid liquid (viscosity determined molecular weight approximately 80,000)prepared in a similar manner to Example 1B in a powder: liquid ratio of3.25: 1. The mix is suitable for the cementation of orthodontic bands asa temporary filling, or for the filling of occlusal fissures as a cariespreventive measure.

EXAMPLE 7 A polyacrylic acid solution is prepared as described inExample 1B but with the following quantities:

Distilled water 90 Ammonium persulphate 0.6 Isopropanol 6 This solutionis maintained at 8085 C. and 30 parts of acrylic acids added over 4hours. The solution is concentrated to 50% polyacrylic acid having aviscosity determined molecular weight approximately 150,000.

This liquid is mixed with a zinc oxide prepared as described in Example1A (c) in a powder liquid ratio of 2: 1. The resulting mix is suitablefor the purposes mentioned in Example 1 and may also be used as thebasis for a periodontal pack.

EXAMPLE 8 TABLE I Settin Compressive time (min. strength (p.s.i.) Cement37 0., Example 100% RH. 24 hr. 7 days EXAMPLE 9 Powder (A) A mixture of90 parts of Analar grade zinc oxide and 10 parts of Analar grademagnesium oxide is heated at 1000 C. for 12 hours. The resulting cake isbroken up and milled and passed through a 300-mesh sieve and then heatedat 1000 C. for 24 hours.

(B) The powder obtained as in (A) is blended with 30 percent by weightof calcium phosphate, Ca (PO0 Polyacrylic acid Solution A A wide-neckedflat-flange-type round-bottomed flask with a S-socket flat-flangeadaptor is fitted with a thermometer, a reflux condenser, gas inlet,liquid inlet and stirrer. The following charge is placed in the flaskand degassed with nitrogen:

Parts Distilled water 200 Ammonium persulphate 2.5

(The ammonium persulphate is assayed before use.)

Solution A Parts Redistilled inhibitor-free acrylic acid 100 Distilledwater 100 Isopropanol 20 Solution B Parts Distilled water 60 Ammoniumpersulphate 2.5

The flask charge is heated to -85 C. while continuing nitrogen purging.Solutions A and B are then added over 2 hours in the ratio of 3.4:1.Continuous addition is desirable but in the absence of metering pumps asatisfactory result has been obtained by adding 10 parts by volume of Ato 2.94 parts by volume of B every 5 minutes. The product is thenmaintained at 80-85 C. for a further two hours.

The solution is vacuum distilled at approximately 20 mm. of mercury(liquid and vapour temperature 80-85 C.) to give a polyacidconcentration of 42%.

The polyacid concentration is determined accurately by dissolving anapproximately 1.5 g. sample in 50 ml. distilled water and titrating withstandardised N sodium hydroxide solution. A potentiometric method ispreferable but phenolphthalein can be used as indicator if care is takennear the end point.

The molecular weight which should be about 20,000 is determined by themethod of Sa'kamoto (Chem. Abstr., 5 8, 13160c). Solutions of thepolyacrylic acid of concentrations of 1 g. of polyacrylic acid/ ml. of 2N sodium hydroxide and below are made and the viscosities measured usinga Size A (l-S centistokes) Ostwald type viscometer at 25.0=0.1 C. Thespecific viscosity is determined graphically in the usual way and themolecular weight calculated from the following equation:

(1 1.05 1O MW,

Polyacrylic acid Solution B The methods of preparation, assay andmolecular weight determination are the same as for the Solution A. Thereactants are as follows in this case:

The solution is concentrated to approximately 42% lgglgggrylic acid asbefore. The molecular weight is about Powder B is slower setting andgives improved strengths compared with Powder A when used with bothpolyacrylate liquids. Present experience suggests that Powder B ispreferred for cementing complex restorations such as bridges, forplacing multiple cavity linings and for other situations where moreWorking time is required.

Solution A may be used particularly for cementing restorations whileSolution B is more suitable for the production of cavity linings.

The powders and solutions may be mixed together preferably in a ratio ofabout 1.5 :1 to 3.5 :1 on a weight basis.

EXAMPLE 10 A polyacrylic acid solution is prepared as in Example 9,Solution A, but with no ammonium persulphate in the flask charge and thetemperature at about 70 C.

The resulting material which has a viscosity determined averagemolecular weight of approximately 170,000 is concentrated to 45% byweight. On mixing with magnesium oxide it forms a slow setting plasticmass suitable for use as a periodontal pack.

EXAMPLE 1 1 A polyacrylic acid solution is prepared in a similar manneras in Example 9A but using approximately 10 parts by weight of ammoniumpersulphate to yield a material with a molecular weight of approximately5000. The resulting very fluid solution can be used at concentrations of40 to 55% by weight with the various metal oxides at powder liquidratios in the range described to give products of generally lowerstrength than those given in Table I. Such materials are usable forvarious dental applications where maximum strength is not desired suchas temporary fillings or cementation.

EXAMPLE 12 A polyacrylic acid solution prepared in a similar manner toExample 9B can be used directly at about 30% by weight concentration togive products with compressive strengths in the region 8000 to 10,000psi and a setting time of 24 hours (in comparison to Table I) which aresuitable for the purposes mentioned in Example 1.

Polyacrylic acid solutions prepared in a similar manner to Examples 9Aand 9B may be used in any concentration in the range 20 to 40% by weightwith zinc oxides prepared as in Example 1 (A), (B) or (C) (but with thequantity of magnesium oxide -10 parts by weight in (C)) provided thatthe metal oxide reactivity is adjusted to give a clinically acceptablesetting time.

These examples are not to be taken as limiting the dental application ofthe invention. For example, the addition of fluorides, bacteriostaticagents and antibiotics in minor amounts may be made to the cement powderas is done with the present dental cements to provide some antibacterialor anticariogenic action. Cotton or asbestos fibres or the like may beadded for greater convenience in manipulation of dressings or packs. Theuse of the surgical cement of the invention is not limited to dentistryand could find application in other forms of surgery, particularlyorthopaedic surgery, where it can be used to assist in the resetting offractured bone material. Many other variations will be evident to thoseexperienced in the formation, manipulation and clinical uses of surgicalcements which can be achieved for particular purposes without affectingthe essential nature of the present invention.

What is claimed is:

1. A process for the preparation of a surgical cement, which comprisesforming a mixture consisting essentially of a surgically acceptablegrade of metal oxide powder, a water-soluble polymer of acrylic acidhaving a viscosity determined average molecular weight of from about5,000 to about 250,000, and water, the amount of the polymer being atleast about 20% by weight of the total weight of the polymer and water,to give a mass that remains plastic long enough to be formed into adesired shape prior to hardening as a surgical cement.

2. The process according to claim 1, in which the mixture consistsessentially of from about 0.5 to about 4 parts by weight of said powderper part by weight of the total weight of the polymer and water.

3. The process according to claim 2, in which the polymer is apolyacrylic acid having a viscosity determined average molecular weightof from about 15,000 to about 150,000 and the amount of the polyacrylicacid is at least about 40% by weight of the total weight of thepolyacrylic acid and water.

4. The process according to claim 2, in which the powder has beendeactivated by heat treatment.

5. The process according to claim 2, in which the powder is a mixtureconsisting predominantly of zinc oxide or zinc oxide together with atleast one additive selected from magnesium oxide, bismuth oxide, calciumphosphate and calcium fluoride.

6. The process according to claim 2, in which the polymer is apoly-acrylic acid having a molecular weight of at least about 25,000,and the amount of the polyacrylic acid is about 50-60% by weight of thetotal weight of the polyacrylic acid and water.

7. The process according to claim 2, in which the polymer is apolyacrylic acid having a viscosity determined average molecular weightof about 25,000, and the amount of the polyacrylic acid is about 42% byweight of the total Weight of the polyacrylic acid and water.

8. The process according to claim 2, in which the polymer is apolyacrylic acid having a viscosity determined average molecular weightof about 55,000-80,000, and the amount of the polyacrylic acid is about42% by weight of the total weight of the polyacrylic acid and water.

9. The process according to claim 2, in which the powder consistsessentially of a mixture containing about by weight zinc oxide and 10%by weight magnesium oxide or a mixture containing 90% by weight zincoxide and 10% by weight magnesium oxide which has been blended with 30%by weight calcium phosphate; said mixture or blend having beendeactivated by heating to about 1000 C.

10. The process according to claim 2, in which the polymer is apolyacrylic acid having a viscosity determined molecular weight of about20,000 or about 55,000, and the amount of the polyacrylic acid is about42% by weight of the total weight of the polyacrylic acid and water.

References Cited UNITED STATES PATENTS 3,655,605 4/1972 Smith.

MELVIN GOLDSTEIN, Primary Examiner US. Cl. X.R.

